Sense amplifier

ABSTRACT

A sense amplifier includes two cross-coupled field effect transistors capacitively coupled to respective bit lines from a memory cell. During a precharging operation two capacitors are charged to precharged voltages indicative of the threshold dispersion voltage between the two transistors. Thereafter, during a sensing operation, the precharged voltages are applied to the gates of the transistors to compensate for the threshold dispersion voltage.

FIELD OF THE INVENTION

The present invention relates to a sense amplifier and more particularly is directed to a sense amplifier for amplifying sensed information signals appearing on bit lines from respective memory cells in a memory.

BACKGROUND OF THE INVENTION

In computer, information-processing and control systems, it is necessary to store digital data and to retrieve it as desired. In a semiconductor memory, an array of storage or memory cells is used, with each memory cell holding one bit of data. When the information can be randomly put into or taken out of each memory cell or element as required, the array is called a random access memory (RAM) which may be static (SRAM) or dynamic (DRAM). The individual memory cells are addressed by data input and output lines, with each memory cell commonly having two output bit lines for indicating the presence of a "0" or "1" bit read out from the memory cell. The "0" and "1" bits are represented by different voltages which, when stored in the memory cells, may be quite small and may accumulate errors tending to reduce the difference between the respective voltages. Therefore, it is advantageous to include sense amplifiers connected to the output bit lines which are adapted to more accurately detect the voltages appearing on the bit lines and to latch the digital bit indicated thereby to provide a more accurate read out.

One advantageous example of such a sense amplifier includes cross-coupled field effect transistors each having a first current carrying electrode (source or drain electrode) coupled to a respective one of the bit lines and a gate electrode coupled to the other of the bit lines. Second current carrying electrodes (drain or source electrodes) of the transistors are coupled together to receive a control signal which permits or prevents turning ON of the transistors. If, for example, NMOS transistors are used, each transistor will turn ON when the difference between the gate voltage and the source voltage is greater than the threshold voltage of the transistor. The signal to be sensed appears on only one of the bit lines, which then carries a voltage higher or lower than the other bit line, depending on the value of the sensed signal. Consequently, when the control signal applied to the connected second current carrying electrodes is lowered to permit turn ON of the two transistors, the transistor having its gate electrode coupled to the bit line carrying the higher voltage will turn ON first. The other transistor will thereafter be maintained in its off state to latch the information read out from the memory cell.

However, the sensitivity of such a sense amplifier depends critically on the threshold voltage V_(th) of each field effect transistor. If the threshold voltage of the transistor which is intended to turn ON first becomes significantly larger than the threshold voltage of the other transistor, it may happen that the other transistor will turn ON instead of the first transistor, resulting in an erroneous read operation. The threshold voltage of a field effect transistor varies in a known way with its channel length, and the development of VLSI integration and miniaturization techniques has made the channel length shorter and shorter and the possible dispersion, or different, in the threshold voltages correspondingly larger and larger. The resulting increased possibility of sensing errors is a significant limitation upon higher integration and miniaturization.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a sense amplifier which removes the above-described difficulties of the prior art.

It is a further object of the present invention to provide a sense amplifier which can reliably sense a read out signal in a highly miniaturized construction.

It is yet a further object of the present invention to provide a sense amplifier which compensates for threshold voltage differences of cross-coupled field effect transistors therein.

In accordance with an aspect of the present invention, a sense amplifier of the type including first and second bit lines for receiving sensed signals and first and second cross-coupled field effect transistors each having a gate electrode and first and second current carrying electrodes comprises capacitive means for capacitively coupling the first current carrying electrodes of the first and second transistors with the second and first bit lines, respectively, first switching means for separably connecting the gate electrodes of the first and second transistors with the first current carrying electrodes of the same transistors, respectively, and second switching means for separably connecting the first current carrying electrodes of the first and second transistors with the first and second bit lines, respectively, the first and second switching means and the second current carrying electrodes of the first and second transistors being responsive to respective control signals for compensating the sense amplifier for threshold voltage differences of the first and second transistors.

In accordance with the present invention, the capacitive means precharges to voltages compensating for a dispersion between the threshold voltages of the first and second transistors during a precharging operation of the sense amplifier prior to its sensing operation and then supplies the precharged voltages to the gate electrodes of the first and second transistors during the sensing operation.

The above and other objects of the present invention will become apparent from reading the following detailed description of a preferred embodiment of the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a sense amplifier according to a preferred embodiment of the present invention;

FIGS. 2(A) to 2(F) are timing diagrams used in explaining the operation of the sense amplifier of FIG. 1;

FIG. 3 is circuit diagram of a conventional sense amplifier; and

FIGS. 4(A) to 4(F) are timing diagrams used in explaining the operation of the sense amplifier of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order that the purposes served by the present invention may be fully appreciated, a sense amplifier 10 according to the prior art will first be described with reference to FIGS. 3 and 4. As shown on FIG. 3, sense amplifier 10 according to the prior art has first and second field effect transistors (FETs) M₁ and M₂ which, in the illustrated example, are NMOS field effect transistors (n-channel FETs). Those skilled in the art will recognize that a corresponding sense amplifier could be constructed using PMOS field effect transistors with appropriate changes in the control signals to be described below. The first transistor M₁ has a first current carrying electrode SD₁₁ connected to a first bit line BL₁ extending from a memory cell (not illustrated). Correspondingly, the second transistor M₂ has a first current carrying electrode SD₂₁ connected to a second bit line BL₂ extending from the memory cell. Transistors M₁ and M₂ also include second current carrying electrodes SD₁₂ and SD₂₂, respectively, which are connected together to receive a common control signal φ_(s). Since transistors M₁ and M₂ are n-channel FETs, the first current carrying electrodes SD₁₁, SD₂₁ are the drain electrodes, while the second current carrying electrodes SD₁₂, SD₂₂ are the source electrodes.

A gate electrode G₁ of transistor M₁ is connected to bit line BL₂, while a gate electrode G₂ of transistor M₂ is connected to bit line BL₁. A reference voltage V_(ref) is supplied to bit line BL₁ through a control transistor Mr₁ which is turned ON and OFF in response to a control signal φ_(r). Reference voltage V_(ref) is also supplied to bit line BL₂ through a second control transistor Mr₂ similarly controlled by control signal φ_(r). Control transistors Mr₁ and Mr₂ are advantageously constructed in the same technology as transistors M₁, M₂, here n-channel FETs.

The operation of sense amplifier 10 according to the prior art will now be described with reference to FIG. 4, wherein the upper line A indicates control signal φ_(s), the middle line B indicates voltage V_(G1), appearing at gate electrode G₁ of transistor M₁, and the lower line C indicates the voltage V_(G2) appearing at gate electrode G₂ of transistor M₂. Control signal φ_(r) (not illustrated) is initially at a high level to maintain transistors Mr₁ and Mr₂ in the ON condition so that V_(ref) is impressed on both bit lines BL₂ and BL₂. For the purpose of this example, let it be assumed that the bit to be read out of the memory cell is a digital "0" having a positive sense voltage V_(s) and is impressed on bit line BL₁ as V_(ref) -V_(s) in accordance with known principles. Sense amplifier 10 is provided for the purpose of sensing sense voltage V_(s) and outputting, for example, at the junction O₁ on FIG. 3, a low voltage V_(L) equal to a standard voltage for a "0" bit in the particular system and which can be recognized as such by other circuit elements. Similarly, sense amplifier 10 will output, at junction O₁ a high voltage V_(H) equal to a standard voltage for a "1" bit in the particular system when sense voltage V_(s) indicating a stored "1" bit is read out from the memory cell and impressed on bit line BL₁ as V_(ref) +V_(s), as discussed below.

As is well known, n-channel FETs such as transistors M₁ and M₂ turn on only when V_(GS) >V_(th), that is, when the gate to source voltage V_(GS), equal to the gate voltage V_(G) minus the source voltage V_(S), is greater than the threshold voltage V_(th) of the respective transistor. Threshold voltage V_(th) is a positive voltage for n-channel FETs, and may be in the range of 2-4 volts. Let it further be assumed initially for this example that transistors M₁ and M₂ have the same threshold voltage V_(th). Ideally, all transistors created on a silicon wafer during the same manufacturing process should have the same threshold voltage V_(th), but in fact the individual threshold voltages V_(th) vary over a range of values which can lead to sensing errors as discussed below.

Initially, as shown in line A of FIG. 4, control signal φ_(s) is at high voltage V_(H) higher than reference voltage V_(ref). Voltage V_(G1) (line B) is the voltage on bit line BL₂, that is, V_(ref), while voltage V_(G2) (line C) is the voltage on bit line BL₁, that is, V_(ref) -V_(s). Consequently,

    V.sub.GS1 =V.sub.G1 -φ.sub.s =V.sub.ref -V.sub.H <O

    V.sub.GS2 =V.sub.G2 -φ.sub.s =V.sub.ref -V.sub.s -V.sub.H <O

and both transistors M₁ and M₂ are OFF.

Subsequently, at time t, a sensing operation of sense amplifier 10 is initiated and control signal φ_(s) begins to decrease towards V_(L). Shortly thereafter, at time t', control signal φ_(s) will have dropped to a value where

    V.sub.GS1 =V.sub.ref -φ.sub.s >V.sub.th

    while

    V.sub.GS2 =V.sub.ref -V.sub.s -φ.sub.s <V.sub.th

so that transistor M₁ turns ON first while transistor M₂ remains OFF. With transistor M₁ ON, current flows from electrode SD₁₁ to electrode SD₁₂ so that, as control signal φ_(s) continues to decrease, the voltage on bit line BL₁ is pulled down to V_(L), as shown in line C of FIG. 4. Moreover, since the voltage on bit line BL₁ is voltage V_(G2), both the gate voltage V_(G2) and the source voltage V(SD₂₂)=φ_(s) of transistor M₂ are pulled down together so that the gate to source voltage V_(GS2) never exceeds V_(th), and transistor M₂ remains OFF throughout the sensing operation. Consequently, the voltage on bit line BL₂ is not pulled down and stays at V_(ref), so that transistor M₁ stays ON. The result is that the digital bit "0" is latched in sense amplifier 10 with transistor M₁ ON and transistor M₂ OFF. This state may be sensed and the low voltage V_(L) at electrode SD₁₁ connected to junction O₁ may be taken out for supply to other circuit elements.

Correspondingly, when a digital "1" is read from the memory cell and impressed on bit line BL₁ as voltage V_(ref) +V_(s), the reduction of control signal φ_(s) toward V_(L) will cause transistor M₂ to turn ON first, since

    V.sub.GS2 =V.sub.G2 -φ.sub.s =V.sub.ref +V.sub.s -φ.sub.s >V.sub.GS1 =V.sub.ref -φ.sub.s.

Now it is the voltage at electrode SD₂₁ which is pulled down because transistor M₂ is ON, while transistor M₁ is held in the OFF state as its gate to source voltage V_(GS1) never exceeds V_(th). Sense amplifier 10 at the end of the sensing operation now has transistor M₁ OFF and transistor M₂ ON, which state is opposite to that when a digital "0" is read out of the memory cell.

The correct operation of sense amplifier 10 is critically dependent on the values of the threshold voltages V_(th) of transistors M₁ and M₂ to ensure that the appropriate transistor will always be turned on first. When the two threshold voltages are equal, the read out sense voltage V_(s) will be correctly sensed regardless of the value of sense voltage V_(s). However, with increasing miniaturization and higher and higher integration, the channel length L of each FET becomes shorter and shorter and therefore the effect of small differences in channel length L becomes more significant. The threshold voltage V_(th) of each FET is dependent on the channel length L, and so the dispersion in threshold voltage is increased as the channel length L is decreased, increasing the risk of sensing errors and thereby limiting further miniaturization.

In particular, using the example of sensing a digital "0", transistor M₁ must always turn ON before transistor M₂, or in other words, the condition for accurate and reliable sensing is

    V.sub.GS2 -V.sub.th2 <V.sub.GS1 -V.sub.th1 always.

    Since

    V.sub.GS2 =V.sub.ref -V.sub.s -φ.sub.s and V.sub.GS1 =V.sub.ref -φ.sub.s,

    the reliable sensing condition is equivalently

    V.sub.ref -V.sub.s -V.sub.th2 <V.sub.ref -V.sub.th1

    or

    V.sub.s >|V.sub.th1 -V.sub.th1 |=ΔV.sub.th.

With increasing miniaturization, ΔV_(th) increases as discussed above, while V_(s), which is proportional to the electrical charge stored in the memory cell and read out therefrom, becomes smaller. Consequently, the likelihood of sensing errors becomes greater and greater and eventually becomes unacceptable.

The present invention eliminates sensing errors due to the dispersion in threshold voltage V_(th) by compensating the gate voltages V_(G1) and V_(G2) applied to gate electrodes G₁ and G₂ of transistors M₁ and M₂. More particularly, in accordance with the invention, the gate electrodes G₁ and G₂ and the bit lines BL₂ and BL₁ are respectively capacitively connected and a voltage corresponding to the threshold dispersion voltage ΔV_(th) is accumulated during a precharging operation prior to the sensing operation for compensating transistors M₁ and M₂ to ensure a highly accurate and reliable sensing operation. To this end, and as shown on FIG. 1, during the precharging operation, the field effect transistors M₁ and M₂ are placed in a common drain configuration so as to effectively to operate as diodes, and capacitors C₁ and C₂ are connected between gate electrodes G₁ and G₂ and bit lines BL₂ and BL₁, respectively, and are charged through such "diodes" to respective precharged voltages. During the sensing operation, transistors M₁ and M₂ are operative in the conventional sense amplifying manner described above with reference to the prior art circuit of FIG. 3, but are compensated by the precharged voltages applied by the respective capacitors C₁ and C₂ to gate electrodes G₁, G₂ to remove the effect of any threshold dispersion voltage ΔV_(th).

More particularly, during the precharging period when transistors M₁ and M₂ are connected in the diode configuration, the voltage across the "diode" from gate electrode G₁ to source electrode SD₁₂ equals threshold voltage V_(th1), while the voltage across the "diode" from gate electrode G₂ to source electrode SD₂₂ of transistor M₂ equals the threshold voltage V_(th2). The capacitors C₁ and C₂, coupled between these "diodes" and the respective bit lines receiving the reference voltage V_(ref) therefore accumulate respective charges equal to the difference between voltage V_(ref) and the voltage of control signal φ_(s) minus V_(th1) and V_(th2), respectively. Thereafter, when transistors M₁ and M₂ are returned to their usual sense amplifying configuration, the difference between the two precharged voltages accumulated on the capacitors corresponds to the threshold dispersion voltage ΔV_(th) =|V_(th1) -V_(th2) |, so that the sensing operation is compensated.

More specifically, as shown on FIG. 1, a sense amplifier 11 according to a preferred embodiment of the present invention has the first capacitor C₁ connected in series between gate electrode G₁ and bit line BL₂ and the second capacitor C₂ connected in series between gate electrode G₂ and bit line BL₁. First switches S₁₂ and S₂₂ separately connect drain electrode SD₁₁ with bit line BL₁ and drain electrodes SD₂₁ with bit line BL₂, respectively, prior to the connections between capacitors C₂ and C₁ and bit lines BL₁ and BL₂ so that, when switches S₁₂ and S₂₂ are open, the capacitors C₁ and C₂ are still connected with bit lines BL₂ and BL₁. First switches S₁₂, S₂₂ are ganged so as to open and close together in response to a control signal φ₂.

Transistor M₁ has its gate electrode G₁ and its drain electrode SD₁₁ separably connected by a switch S₁₁ connected before the series connection of gate electrode G₁ to capacitor C₁. Correspondingly, transistor M₂ has its gate electrode G₂ separably connected to its drain electrode SD₂₁ by a switch S₂₁ connected before the series connection of gate electrode G₂ to capacitor C₂. Switches S₁₁ and S₂₁ are ganged and so as to open and close together in response to a control signal φ₁. When switches S₁₁ nd S₂₁ are closed, the gate and drain electrodes of the n-channel FETs are connected in a common drain configuration and function equivalently to diodes with the forward voltage taken from gate to source. Switches S₁₁ S₁₂, S₂₁ and S₂₂ may all be constructed as control transistors similar to the transistors Mr₁ and Mr₂, and are "open" when OFF and "closed" when ON. It will be appreciated that, apart from the added capacitors C₁ and C₂ and switches S₁₁,S₁₂,S₂₁ and S₂₂, the sense amplifier 11 embodying the invention is similar to the previously described sense amplifier 10 according to the prior art and has its corresponding parts identified by the same references.

The operation of sense amplifier 11 embodying the invention will now be described with reference to FIG. 2. Voltage V_(A) (line E in FIG. 2) is the voltage taken at the connection point between capacitor C₁ and gate electrode G₁, while voltage V_(B) is the voltage taken at the connection point between capacitor C₂ and gate electrode G₂ . Prior to time t₀, that is, prior to the precharging operation of sense amplifier 11, control signal φ₁ (line A) in FIG. 2 is at a low level to open switches S₁₁ and S₂₁, control signal φ₂ (line B) in FIG. 2 is at a high level to close switches S₁₂ and S₂₂, control signal φ_(s) (line D) in FIG. 2 is at high voltage V_(H) so that transistors M₁ and M₂ are OFF and control signal φ_(r) (line C) in FIG. 2 is at a low level to turn OFF control transistors Mr₁ and Mr₂. As control signal φ_(r) rises to its high level, it turns ON control transistors Mr₁ and Mr₂ to supply reference voltage V_(ref) to bit lines BL₁ and BL₂ (line F) on FIG. 2. Since transistors M₁ and M₂ remain OFF, voltages V_(A) and V_(B) are undefined at this time. Thereafter, at time t₀, to start the precharging operation, control signal φ₁ goes to its high level from its low level to close switches S₁₁ and S₂₁ and thereby connect gate electrodes G₁ and G₂ to bit lines BL₁ and BL₂, through closed switches S₁₂, S₂₂ respectively. Consequently, voltages V_(A) and V_(B) both go to V_(ref).

At time t₁, control signal φ₂ goes from its high level to its low level, opening switches S₁₂ and S₂₂ to break the connection between drain electrodes SD₁₁ and SD₂₁ and bit lines BL₁ and BL₂, respectively. This places transistors M₁ and M₂ into their diode configurations, so that they operate equivalently as diodes connected in series with capacitors C₁ and C₂, respectively. Since the voltages on bit lines BL₁ and BL₂ and voltages V_(A) and V_(B) are all V_(ref), capacitors C₁ and C₂ do not charge. Since control signal φ_(s) is still at the high level of voltage V_(H) greater than V_(ref), the "diodes" are reversed biased and no current flows.

At time t₂, control signal φ_(s) is reduced to an intermediate low voltage V_(L) ' which is higher than low voltage V_(L) but lower than V_(ref) so as to permit the "diodes" to be forward biased. Consequently, voltages corresponding to threshold voltages V_(th1) and V_(th2) of transistors M₁ and M₂, respectively are applied to capacitors C₁ and C₂. In particular, voltage V_(A) at gate electrode G₁ goes to a voltage V_(L) '+V_(th1), that is, the voltage V_(L') at the anode of the "diode" plus the diode voltage V_(th1). Correspondingly, the voltage V_(B) at gate electrode G₂ goes to V_(L) '+V_(th2). Consequently, the voltage difference between voltage V_(A) at one terminal of capacitor C₁ and voltage V_(ref) at bit line BL₂ at the other terminal of capacitor C₁, i.e. the voltage to which capacitor C₁ precharges, is

    V.sub.C1 =V.sub.ref -(V.sub.L '+V.sub.th1)

Correspondingly, the voltage difference across the two terminals of capacitor C₂, i.e. the voltage to which capacitor C₂ precharges, is

    V.sub.C2 =V.sub.ref -(V.sub.L '+V.sub.th2).

As is readily apparent, the magnitude of the difference between the two precharged voltages is |V_(th1) -V_(th2) |, that is, the dispersion threshold voltage ΔV_(th).

At time t₃, control signal φ_(s) is changed from V_(L) ' to an intermediate high voltage level V_(H) ' slightly lower than high voltage V_(H). This temporarily reverse biases the "diodes" while switches S₁₁, S₂₁ are opened and switches S₁₂ and S₂₂ are closed, as discussed below, to prevent any noise spikes generated by the opening and closing of these switches from discharging capacitors C₁, C₂. If noise spikes are otherwise removed from the circuit, this change in control signal φ_(s) is unnecessary.

At time t₄, control signal φ₁ goes from its high level to its low level to open switches S₁₁ and S₂₁ so as to disconnect gate electrode G₁ from drain electrode SD₁₁ of transistor M₁ and gate electrode G₂ from drain electrode SD₂₁ of transistor M₂, respectively. Transistors M₁ and M₂ are no longer in their diode configuration and return to conventional transistor operation.

At time t₅, control signal φ₂ goes from its low level to its high level to again close switches S₁₂ and S₂₂ for connecting drain electrode SD₁₁ of transistor M₁ to bit line BL₁ and connecting drain electrode SD₂₁ of transistor M₂ to bit line BL₂. Sense amplifier 11 is now configured identically to sense amplifier 10 of FIG. 3 except for the presence of capacitors C₁ and C₂ carrying precharged voltages V_(C1) and V_(C2), respectively.

At time t₆, control signal φ_(r) goes from its high level to its low level to turn OFF control transistors Mr₁ and Mr₂ and to cut off the supply of V_(ref) to bit lines BL₁ and BL₂. During the period from t₃ through t₆ up to the start of a sensing operation at a time t₇, the precharged voltages V_(Cl) =V_(ref) -(V_(L'+V) _(th1)) and V_(C2) =V_(ref) -(V_(L) '+V_(th2)) are maintained on capacitors C₁ and C₂, respectively, without alteration. These voltages V_(C1) and V_(C2) are applied to gate electrodes G₁ and G₂, respectively, during the sensing operation of sense amplifier 11, as will now be described.

At time t₇, a sensing operation begins and, in response to turn on of an access transistor (not shown) at a select line of the memory cell to be read, the electrical charge stored in a capacitative element of the selected memory cell appears as a sense voltage V_(s) at bit line BL₁. In the present example, it is assumed that the bit stored in the memory cell is a digital "0", and so the voltage of bit line BL₁ is V_(ref) -V_(s), as in the example described above in connection with conventional sense amplifier 10. Similarly, the potential of bit line BL₂ is maintained at V_(ref). As discussed above, if the bit stored in the memory cell were a digital "1" the voltage impressed on bit line BL₁ would be V_(ref) +V_(s).

When voltage V_(ref) -V_(s) appears on bit line BL₁, capacitor C₂ cannot discharge through transistor M₂, which is maintained in its OFF state by the intermediate high voltage V_(H) ', of control signal φ_(s) appearing at its source electrode SD₂₂. Therefore, voltage V_(C2) cannot change to compensate for the change in the voltage on bit line BL₁ from V_(ref) to V_(ref) -V_(s), and so voltage V_(B) appearing at gate electrode G₂ must change instead. In particular,

    V.sub.B =V.sub.ref -V.sub.s -(V.sub.ref -[V.sub.L '+V.sub.th2)]=V.sub.L '+V.sub.th2 -V.sub.s.

At the same time, the voltage at bit line BL₂ remains at V_(ref), so that voltage V_(A) at gate electrode G₁ is

    V.sub.A =V.sub.ref -[V.sub.ref -(V.sub.L '+V.sub.th1)]=V.sub.L '+V.sub.th1

In other words, voltages V_(A) and V_(B) are higher than the voltages which would appear in conventional sense amplifier 10 by the respective threshold voltages V_(th1) and V_(th2). Therefore, the gate to source voltages V_(GS1) and V_(GS2) which determine when transistors M₁ and M₂ are respectively turned ON are fully compensated by the respective threshold voltages V_(th1) and V_(th2), and sense amplifier 11 has extremely high sensitivity.

Thus, when, at a time t₈, control signal φ_(s) is lowered from intermediate high voltage V_(H) ' toward low voltage V_(L) to start the latching of transistors M₁ and M₂, gate to source voltage V_(GS1) between gate electrode G₁ and source electrode SD₁₂ is ##EQU1## while the gate to source voltage V_(GS2) between gate electrode G₂ and source electrode SD₂₂ of transistor M₂ is ##EQU2## It is readily apparent that the relation

    VGS.sub.1 >VGS.sub.2

    is equivalent to

    V.sub.A -φ.sub.s -V.sub.th1 >V.sub.B -φ.sub.s -V.sub.th2

    which is, in turn equivalent to

    V.sub.L '-φ.sub.s >V.sub.L '-φ.sub.s -V.sub.s

and holds for all values of control signal φ_(s) from V_(H) ' to V_(L). Therefore, transistor M₁ will always be turned ON first and the effect of the dispersion of the threshold voltages V_(th1) and V_(th2) is compensated. Thereafter, in a manner identical to the described operation of the known sense amplifier 10, when transistor M₁ is turned ON, the voltage at drain electrode SD₁₁ is pulled down to low voltage V_(L) as control signal φ_(s) is reduced to that low voltage V_(L), and voltage V_(B) is similarly lowered, since transistor M₂ remains OFF so that capacitor C₂ cannot discharge therethrough. Thus, the voltage appearing at bit line BL₁ is reduced to the low voltage V_(L) while the voltage at bit line BL₂ is maintained at V_(ref) to complete the latching operation.

In accordance with the present invention, sense amplifier 11 accumulates, at capacitors C₁ and C₂, precharged voltages corresponding to the threshold dispersion voltage ΔV_(th) of the threshold voltages V_(th1) and V_(th2) of transistors M₁ and M₂, so that, during the sensing operation this threshold dispersion voltage ΔV_(th) is compensated for completely. Consequently, a manufacturing process may be employed for providing large numbers of chips including sense amplifiers according to the present invention on a single wafer with the assurance that even those sense amplifiers having significant threshold voltage dispersions will function acceptably. Therefore, the yield of such process, defined as the number of good chips on the wafer divided by the total number of chips on the wafer, will be increased. Furthermore, since the compensation for the threshold dispersion voltage V_(th) is performed dynamically during each precharging operation prior to each sensing operation, the continuing accuracy of the sense amplifiers is not so heavily dependent upon precision in the manufacturing process, so that the sense amplifiers according to the present invention have a high time dependent reliability, defined as the number of devices still operating acceptably after a period of time divided by the total number of devices originally produced.

Even when the capacitance within each memory cell is reduced, for example by a factor of 5 or 6, as further miniaturization is achieved, a reliable operation free of malfunction is available due to the high sensitivity of the sense amplifier in accordance with the present invention. Advantageously, an excellent result may be obtained by applying the sense amplifier according to the present invention to a memory device fabricated using a silicon-on-insulator (SOI) structure.

Although the above description has been given for a preferred embodiment using NMOS technology, it will be apparent that an equivalent sense amplifier could be constructed using PMOS technology, with suitable inversion of the control signals and voltage reference levels in accordance with conventional principles.

Although a preferred embodiment of the present invention has been described in detail with reference to the drawings, it will be apparent that the invention is not limited to the precise embodiment, and that many modifications and variations may be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined by the appended claims. 

I claim:
 1. A sense amplifier of the type including first and second bit lines for receiving sensed signals and first and second cross-coupled field effect transistors each having a gate electrode and first and second current carrying electrodes, comprising;capacitive means for capacitively coupling said gate electrodes of said first and second transistors with said second and first bit lines, respectively; first switching means for separably connecting said gate electrodes of said first and second transistors with the first current carrying electrodes of said first and second transistors, respectively; and second switching means for separably connecting said first current carrying electrodes of said first and second transistors with said first and second bit lines, respectively; said first and second switching means and said second current carrying electrodes of said first and second transistors being responsive to respective control signals for compensating said sense amplifier for threshold voltage differences of said first and second transistors.
 2. A sense amplifier according to claim 1, wherein said capacitive means supplies voltages compensating for a dispersion between the threshold voltages of said first and second transistors to said gate electrodes during a sensing operation of said sense amplifier.
 3. A sense amplifier according to claim 2, wherein said first and second switching means are responsive to the respective control signals for precharging said capacitive means to precharged voltages prior to said sensing operation of said sensed amplifier, said capacitive means supplying the precharged voltages to said gate electrodes during said said sensing operation.
 4. A sense amplifier according to claim 3, wherein said capacitive means includes a first capacitor connected between the gate electrode of said first transistor and said second bit line and a second capacitor connected between said gate electrode of said second transistor and said first bit line.
 5. A sense amplifier according to claim 3, wherein said second current carrying electrodes of said first and second transistors are coupled together to receive a common control signal to permit or prevent turn on of said transistors, said precharged voltages are determined such that when said common control signal permits turn on, the difference between the voltage appearing at the gate electrode of said first transistor and the threshold voltage thereof differs from the difference between the voltage appearing at the gate electrode of said second transistor and the threshold voltage thereof by an amount equal to a sensed signal appearing on a selected one of said bit lines, whereby a selected one of said first and second transistors will always turn on before the other of said transistors in dependence on the sensed signal. 